This invention deals generally with antennas and more specifically with an antenna with significantly lower height above ground than those usually available.
A single quarter wavelength vertical monopole antenna may be the classic of all antenna designs. We see it for virtually every AM broadcasting station, and we see approximations of it on almost every automobile in the rod attached to an upper surface of the automobile. Such an antenna is an unbalanced antenna, and for transmitting it must be fed against a ground plane, a relatively flat conductive surface. The impedance of such an antenna is approximately 36 ohms at resonance.
For use in AM medium wave broadcasting the quarter wavelength antenna must be 447 feet (136 meters) high at 550 kHz, and at 1600 kHz it is still 154 feet (47 meters) high. Such heights not only require towers demanding considerable design and expense, but often such high towers are restricted by local building codes or pose a danger to aircraft.
One solution to the problem of high antennas was proposed as early as 1909 by Simon Eisenstein of Kiev, Russia. This was an antenna with a single vertical conductor and top loading. For the top loading, a horizontal plane formed of multiple radial conductors is added atop the vertical section that has a height much less than a quarter wavelength. Unfortunately, however, this configuration produces an antenna with a very low impedance if the height is in the desirable range of less than 0.1 wavelength.
It would be very beneficial, particularly to broadcasters and for vehicle antenna users, to have an antenna design which has both a very low height, that is, less than 0.1 wavelength, and also could be matched to a typical 50 ohm impedance system.
The present invention is a low profile antenna configuration constructed of multiple top loaded approximately vertical upright conductors clustered together. In the preferred embodiment, in order to yield an omnidirectional pattern, the individual conductors are spaced along a small circular pattern, with each conductor being excited by only an appropriate fraction of the total signal power which would be applied to a single monopole antenna for the same application. Thus a four conductor pattern has the upright conductors spaced apart at 90 degree separation points, and for a transmitting antenna configuration, each conductor is fed one-quarter of the total power.
Another feature of the invention is that the top loading on each upright conductor is only a single conductor extending away from the cluster of upright conductors, and the length of the top loading conductors and the heights of the their ends above the ground plane can be adjusted to achieve resonance and to provide the desired impedance. For ease of construction, the upright sections and the non-vertical extension can be a continuous conductor bent to form both the upright section and the non-vertical extension. With the further use of impedance matching sections on each feed to an upright conductor, a high enough impedance can be achieved on each individual conductor feed to result in an impedance which matches the commonly used 50 ohm impedance systems when all the individual antennas are fed in parallel.
The present invention thereby furnishes low profile, very low height, antenna configurations which can match the typical signal feed cables and yield nearly uniform omnidirectional radiation fields substantially identical to the fields provided by quarter wavelength monopole antennas.